As a representative example of a working machine in which a plurality of working elements are hydraulically driven, there are construction machines such as a hydraulic excavator, a hydraulic crane and so on. Each of these construction machines is equipped with a hydraulic drive system which is provided with a plurality of actuators for driving the plurality of working elements. In the hydraulic drive system, driving of the plurality of actuators is controlled by a plurality of directional control valves for controlling flows of hydraulic fluid delivered from a hydraulic pump.
By the way, as the hydraulic drive system of this kind, there is a system called a load sensing system in which, as disclosed, for example, in JP,A,60-11706, a pump delivery flow rate is controlled such that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure of a plurality of actuators (a load pressure of a corresponding actuator in the case of individual operation) is retained to a prescribed value, so that the flow rate required for driving the actuators is delivered from the hydraulic pump. In the load sensing system, it is preferable to use a directional control valve of closed center type in which the spool closes the respective ports at a neutral position to prevent flow of the hydraulic fluid. Thus, unnecessary discharge of hydraulic fluid at the time each directional control valve is in the neutral position is minimized so that unloading operation of the hydraulic pump is made possible in combination with an unloading valve.
On the other hand, the plurality of directional control valves used in the hydraulic drive system are, in practice, formed as a valve apparatus in which the directional control valves are arranged collectively. The conventional valve apparatus having the directional control valves of closed center type are known from, for example, see JP,A,49-19282 and JP,A,54-59575. The conventional valve apparatus comprises a casing formed therein with a plurality of parallel spool bores arranged in a single row, a single common pump port, at least one tank port and a plurality of actuator ports with each pair associated with a single spool bore, and a plurality of spools slidably inserted respectively in the spool bores in the casing. The spools form respectively a plurality of directional control valves of closed center type, and each spool controls communication between the common pump port and tank port and the corresponding pair of the actuator ports. The common pump port communicates with a common pump line which extends perpendicularly to the plurality of parallel spool bores arranged in the single row, and successively passes through the spool bores.
However, the above-described conventional valve apparatus has the following problems.
The conventional valve apparatus is arranged such that the common pump line extends perpendicularly to the plurality of parallel spool bores arranged in the single row, and successively passes through the spool bores. For this reason, when the hydraulic fluid flows into the spools in the bores through the pump line, the hydraulic fluid surely passes through narrow grooves around one or more spools located upstream of the spool remotest from the pump port. Accordingly, pressure losses are large and cause an energy loss. Further, since temperature is raised due to an increase In heat generation, the reliability of the valve apparatus is deteriorated.
Furthermore, since the pressure losses are caused due to a passing of the hydraulic fluid through the spools, the more the spools are positioned downstream, the more the pressure losses increase. For this reason, in the case where the valve apparatus is used in the load sensing system, since a differential pressure between the pump delivery pressure and the load pressure is retained at a prescribed value, differential pressures across the downstream spools having their respective large pressure losses are reduced so that, even if the spools are operated at their respective maximum openings, sufficient passing flow rate cannot correspondingly be obtained. Thus, the actuators cannot be driven at the maximum speed, so that operability is reduced. Further, since the pressure losses are not constant for all of the spools in the case where the plurality of actuators are driven simultaneously, a difference in the differential pressure across each spool occurs in accordance with a difference in the pressure losses. For this reason, the characteristic of the passing flow rate with respect to the operational amounts of the respective spools differs spool by spool, so that a problem occurs in the operability of the simultaneous driving of the plurality of actuators.
Moreover, in the case where the pair of spools of the valve apparatus control left- and right-hand traveling motors of a construction vehicle as directional control valves, the differential pressures are different from each other between the pair of spools as described above. Accordingly, the flow rates of the hydraulic fluid supplied respectively to the left- and right-hand traveling motors are different from each other so that the traveling direction is brought to such a condition as to be bent naturally. That is, it becomes difficult to secure linearity of the traveling.
It is an object of the invention to provide a valve apparatus having a plurality of directional control valves of closed center type arranged collectively and a hydraulic drive system provided with the valve apparatus, in which, a pressure loss between a pump port and each of a plurality of spools is relatively small.
It is another object of the invention to provide a valve apparatus having a plurality of directional control valves of closed center type arranged collectively and a hydraulic drive system provided with the valve apparatus, in which the pressure losses between a pump port and a plurality of spools are substantially the same for all of the spools to improve the operability.
It is still another object of the invention to provide a valve apparatus having a plurality of directional control valves of closed center type arranged collectively and a hydraulic drive system provided with the valve apparatus, in which it is possible to secure linearity of traveling when left- and right-hand motors are controlled.